Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. In recent years, applicability of secondary batteries has been realized as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs). As such, a strong need for the stability of batteries due to increase in the number and capacity constituting a battery pack has grown. In addition, when the battery is installed in a vehicle, exposures to the vibration and external impact are increased. Thereby, a mechanical strength of batteries that can operate as a resistivity against external impact is also demanded as a major characteristic. In the light of such trends, a great deal of research and study has been focused on secondary batteries which are capable of meeting various demands. Among other things, there has been an increased demand for lithium secondary batteries having high-energy density, high-discharge voltage and power output stability.
The lithium secondary battery uses a metal oxide such as LiCoO2 as a cathode active material and a carbonaceous material as an anode active material, and is prepared by disposition of a porous polymer separator between the anode and cathode and the addition of a non-aqueous electrolyte containing a lithium salt such as LiPF6. Upon charging, lithium ions exit from the cathode active material and migrate to enter into a carbon layer of the anode. In contrast, upon discharging, lithium ions exit from the carbon layer and migrate to enter into the cathode active material. Here, the non-aqueous electrolyte serves as a medium through which lithium ions migrate between the anode and cathode. Such a lithium secondary battery must be basically stable in a range of operating voltage of the battery and must have ability to transfer ions at a sufficiently rapid rate.
The non-aqueous electrolyte is injected into the battery at the final stage of fabricating the lithium secondary battery. At this time, the electrodes are rapidly and completely wetted by the electrolyte so as to reduce time consumption for the battery fabrication and to optimize the battery performances.
An aprotic organic solvent, such as ethylene carbonate, diethyl carbonate, or 2-methyl tetrahydrofuran, is mainly used as the non-aqueous electrolyte of the lithium secondary battery. Such an electrolyte is a polar solvent having polarity enough to effectively dissolve and dissociate the electrolyte salt, and, at the same time, an aprotic solvent having no active hydrogen. Occasionally, this electrolyte has high viscosity and surface tension due to wide interaction in the electrolyte. Consequently, the non-aqueous electrolyte of the lithium secondary battery has a low affinity for an electrode material including a polytetrafluoroethylene and polyvinylidene fluoride bonding agent, and, as a result, the electrode material is not easily wetted by the non-aqueous electrolyte. This is one of the principal factors to ineffectively increase the time consumption for the battery fabrication.
Especially, an anode used in the lithium secondary battery is strongly oleophilic, thus its wettability by the hydrophilic electrolyte is not good. When the activating operation of the battery is carried out while the electrodes are not sufficiently wetted by the electrolyte, a solid electrolyte interface (SEI) film is not properly formed at the anode, and therefore, the life characteristics of the battery is deteriorated.
In addition, as high-capacity batteries are in demand, lithium secondary batteries with the higher energy density of electrode are being developed. However, the improvement in the energy density led to a great decrease in the electrode porosity, and, as a result, difficulties to penetrate electrolyte into the electrode increased. When the active materials constituting the electrodes are not sufficiently wetted by the electrolyte, a path for migrating lithium ions is restricted, thereby causing the problems such as deterioration in the rate characteristics and reduction in the capacity. Therefore, electrode components with excellent wettability by the electrolyte are in demand.
Therefore, there is an urgent need for a technology that can increase electrode wettability by the electrolyte and improve stability, while having excellent performance of batteries.
In this connection, the present invention, as described below, provides an electrode material containing a clay mineral for improving stability of a battery, and, at the same time, wettability by the electrolyte.
There has not existed a technique to contain a clay mineral in an electrode material to this point. However, some techniques for using cay minerals as electrode active materials or coating clay minerals onto electrode active materials are known. For example, Japanese Patent Laid-Open Publication No. 1997-115505 discloses a technique for covering the surface of a positive electrode material with a lithium conductive clay material for preventing generation of self-discharge and decomposition of an electrolyte as a result of the electrolyte contacting and reacting with the positive electrode material. Japanese Patent Laid-Open Publication No. 2004-296370 is a technique that uses a layered clay mineral as an anode active material, and discloses a technique for manufacturing the anode active material by separating a layered clay mineral with injected lithium ions between the layers of the clay mineral. In addition, Japanese Patent No. 3587935 discloses a technique that uses the carbon laminated body as a negative electrode active material prepared by inserting carbon atoms into a layered clay mineral such as saponite or montmorillonite, heat treating for polymerization, and then performing a carbonization process at 500 to 1200° C.
However, the above-mentioned techniques use only the swelling layered clay minerals, in which the clay mineral is used as an anode active material for improving discharge characteristics or coated on a cathode active material for preventing an electrolyte from decomposition. Therefore, there is a big difference with the present invention where a clay mineral is added to an electrode material so as to improve wettability by an electrolyte.
Meanwhile, Japanese Patent Laid-Open Publication No. 1996-279354 discloses a technique for manufacturing an electrode for secondary battery for improving an initial charge/discharge characteristic, containing a conductive polymer as an active material and at least one kind of swelling layer-shaped clay compound in an electrode. The technique is on the assumption that the application thereof is limited to electrodes using conductive polymer as the electrode active material, and more excellent characteristics are exerted when a swelling layer-shaped clay compound having an oleophilic surface is used.
However, when using a conductive polymer as an electrode, the stability is very low compared with other inorganic electrode materials. Thus, the conductive polymer is not suitable for the application to the batteries which demand a long lifespan. Moreover, upon adding a swelling clay mineral into the conductive polymer, a texture is sparsely formed by the largely swelled clay mineral. Such texture sparsely formed induces change in the configuration of the conductive polymer due to the pressure and contraction applied during the repetitive contraction and expansion of electrodes upon charging/discharging. Consequently, a prolonged lifespan and stability of the battery is deteriorated even more. In addition, by adding a clay mineral into a conductive polymer, the conductive polymer exhibits high rigidity. Thus, there are many problems such as a partial degradation of the conductive polymer due to an external impact.